In conventional TFT liquid crystal panels, the reversal of a liquid crystal element is controlled by applying an analog voltage to a pixel electrode using a D/A converting source driver. As a result of upsizing, such liquid crystal panels have headaches in, for instance, moving image characteristics (speed of response), viewing angle, luminance shifting and angle shifting of colors, V-T accuracy, and uniformity of in-plane luminance distribution. These problems are caused by the following two electrical issues.
The capacitive driving force and the accuracy of an output of a source driver are the first issue.
The second issue is, as in FIG. 9, that voltage characteristics greatly differ between pixels at different positions (between a pixel (pixel 1) near the source driver and a pixel (pixel 2) far from the source driver). That is, when a single color is displayed on a whole liquid crystal panel, i.e. when identical signals are supplied to all pixels, even if identical voltages should have been applied to pixels at different positions (i.e. pixels 1 and 2) with little time difference, in reality different voltages are applied to the respective pixels. As a result, the rise of the pixel 2 is not immediately carried out so that the drive period of liquid crystal in the pixel 2 is shortened, and the pixel 2 is not sufficiently charged.
To resolve these issues, Japanese Laid-Open Patent Application No. 7-261155/1995 (Tokukaihei 7-261155; published on Oct. 13, 1995), U.S. Pat. No. 6,335,778 (registered on Jun. 1, 2002) corresponding to Japanese Laid-Open Patent Application No. 10-68931 (Tokukaihei 10-68931; published on Mar. 10, 1998), and Japanese Laid-Open Patent Application No. 6-138844 (Tokukaihei 6-138844; published on May 20, 1994) teach the adoption of an area ratio gray scale against a liquid crystal display. According to these publications, one pixel includes a plurality of sub-pixels and the tone of the pixel is determined by the number of electrodes in the sub-pixels being turned on. In this manner, since binary driving is carried out when the area ratio gray scale is adopted, the first issue can be resolved.
However, the binary-driving liquid crystal panel and an analog liquid crystal panel are identical to the extent that a signal voltage is applied to pixel electrodes via source lines. Thus, as in the case of a liquid crystal panel to which an analog voltage is applied, pixels at different positions receive different voltages so that the time necessary for the rise and the amount of change are different between these pixels. In short, the second issue cannot be resolved. The time difference of the drive of liquid crystal between pixels at different positions occurs because the pixels at different positions are not equidistant from the source driver. Furthermore, the difference of voltages applied to the respective pixels at different positions occurs because the attenuation of a source drive voltage applied to the pixels at different positions, as a result of RC components of source lines, varies in accordance with the length of the source line. There have been attempts to improve the speed of response of a liquid crystal panel by means of graphic data processing (overshoot), but it has been difficult to determine the amount of compensation in view of, for instance, the variation of the speed of reversal due to the temperature variation of the liquid crystal. Also, when the liquid crystal display adopting the area ratio gray scale reproduces a low-luminance image, the image appears unnatural and jaggy due to the pixels, i.e. pixels appear to be distanced from each other. For this reason, it has been difficult to reproduce smooth images by a liquid crystal display.
The present invention is done to solve the above-described problem. The objective of the present invention is to provide a liquid crystal display which can reproduce smooth images.